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Diffstat (limited to '')
| -rw-r--r-- | library/std/src/sys/unix/locks/pthread_rwlock.rs | 173 |
1 files changed, 173 insertions, 0 deletions
diff --git a/library/std/src/sys/unix/locks/pthread_rwlock.rs b/library/std/src/sys/unix/locks/pthread_rwlock.rs new file mode 100644 index 000000000..adfe2a883 --- /dev/null +++ b/library/std/src/sys/unix/locks/pthread_rwlock.rs @@ -0,0 +1,173 @@ +use crate::cell::UnsafeCell; +use crate::mem::forget; +use crate::sync::atomic::{AtomicUsize, Ordering}; +use crate::sys_common::lazy_box::{LazyBox, LazyInit}; + +pub struct RwLock { + inner: UnsafeCell<libc::pthread_rwlock_t>, + write_locked: UnsafeCell<bool>, // guarded by the `inner` RwLock + num_readers: AtomicUsize, +} + +pub(crate) type MovableRwLock = LazyBox<RwLock>; + +unsafe impl Send for RwLock {} +unsafe impl Sync for RwLock {} + +impl LazyInit for RwLock { + fn init() -> Box<Self> { + Box::new(Self::new()) + } + + fn destroy(mut rwlock: Box<Self>) { + // We're not allowed to pthread_rwlock_destroy a locked rwlock, + // so check first if it's unlocked. + if *rwlock.write_locked.get_mut() || *rwlock.num_readers.get_mut() != 0 { + // The rwlock is locked. This happens if a RwLock{Read,Write}Guard is leaked. + // In this case, we just leak the RwLock too. + forget(rwlock); + } + } + + fn cancel_init(_: Box<Self>) { + // In this case, we can just drop it without any checks, + // since it cannot have been locked yet. + } +} + +impl RwLock { + pub const fn new() -> RwLock { + RwLock { + inner: UnsafeCell::new(libc::PTHREAD_RWLOCK_INITIALIZER), + write_locked: UnsafeCell::new(false), + num_readers: AtomicUsize::new(0), + } + } + #[inline] + pub unsafe fn read(&self) { + let r = libc::pthread_rwlock_rdlock(self.inner.get()); + + // According to POSIX, when a thread tries to acquire this read lock + // while it already holds the write lock + // (or vice versa, or tries to acquire the write lock twice), + // "the call shall either deadlock or return [EDEADLK]" + // (https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_rwlock_wrlock.html, + // https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_rwlock_rdlock.html). + // So, in principle, all we have to do here is check `r == 0` to be sure we properly + // got the lock. + // + // However, (at least) glibc before version 2.25 does not conform to this spec, + // and can return `r == 0` even when this thread already holds the write lock. + // We thus check for this situation ourselves and panic when detecting that a thread + // got the write lock more than once, or got a read and a write lock. + if r == libc::EAGAIN { + panic!("rwlock maximum reader count exceeded"); + } else if r == libc::EDEADLK || (r == 0 && *self.write_locked.get()) { + // Above, we make sure to only access `write_locked` when `r == 0` to avoid + // data races. + if r == 0 { + // `pthread_rwlock_rdlock` succeeded when it should not have. + self.raw_unlock(); + } + panic!("rwlock read lock would result in deadlock"); + } else { + // POSIX does not make guarantees about all the errors that may be returned. + // See issue #94705 for more details. + assert_eq!(r, 0, "unexpected error during rwlock read lock: {:?}", r); + self.num_readers.fetch_add(1, Ordering::Relaxed); + } + } + #[inline] + pub unsafe fn try_read(&self) -> bool { + let r = libc::pthread_rwlock_tryrdlock(self.inner.get()); + if r == 0 { + if *self.write_locked.get() { + // `pthread_rwlock_tryrdlock` succeeded when it should not have. + self.raw_unlock(); + false + } else { + self.num_readers.fetch_add(1, Ordering::Relaxed); + true + } + } else { + false + } + } + #[inline] + pub unsafe fn write(&self) { + let r = libc::pthread_rwlock_wrlock(self.inner.get()); + // See comments above for why we check for EDEADLK and write_locked. For the same reason, + // we also need to check that there are no readers (tracked in `num_readers`). + if r == libc::EDEADLK + || (r == 0 && *self.write_locked.get()) + || self.num_readers.load(Ordering::Relaxed) != 0 + { + // Above, we make sure to only access `write_locked` when `r == 0` to avoid + // data races. + if r == 0 { + // `pthread_rwlock_wrlock` succeeded when it should not have. + self.raw_unlock(); + } + panic!("rwlock write lock would result in deadlock"); + } else { + // According to POSIX, for a properly initialized rwlock this can only + // return EDEADLK or 0. We rely on that. + debug_assert_eq!(r, 0); + } + *self.write_locked.get() = true; + } + #[inline] + pub unsafe fn try_write(&self) -> bool { + let r = libc::pthread_rwlock_trywrlock(self.inner.get()); + if r == 0 { + if *self.write_locked.get() || self.num_readers.load(Ordering::Relaxed) != 0 { + // `pthread_rwlock_trywrlock` succeeded when it should not have. + self.raw_unlock(); + false + } else { + *self.write_locked.get() = true; + true + } + } else { + false + } + } + #[inline] + unsafe fn raw_unlock(&self) { + let r = libc::pthread_rwlock_unlock(self.inner.get()); + debug_assert_eq!(r, 0); + } + #[inline] + pub unsafe fn read_unlock(&self) { + debug_assert!(!*self.write_locked.get()); + self.num_readers.fetch_sub(1, Ordering::Relaxed); + self.raw_unlock(); + } + #[inline] + pub unsafe fn write_unlock(&self) { + debug_assert_eq!(self.num_readers.load(Ordering::Relaxed), 0); + debug_assert!(*self.write_locked.get()); + *self.write_locked.get() = false; + self.raw_unlock(); + } + #[inline] + unsafe fn destroy(&mut self) { + let r = libc::pthread_rwlock_destroy(self.inner.get()); + // On DragonFly pthread_rwlock_destroy() returns EINVAL if called on a + // rwlock that was just initialized with + // libc::PTHREAD_RWLOCK_INITIALIZER. Once it is used (locked/unlocked) + // or pthread_rwlock_init() is called, this behaviour no longer occurs. + if cfg!(target_os = "dragonfly") { + debug_assert!(r == 0 || r == libc::EINVAL); + } else { + debug_assert_eq!(r, 0); + } + } +} + +impl Drop for RwLock { + #[inline] + fn drop(&mut self) { + unsafe { self.destroy() }; + } +} |